Best is subjective.
Other than capacity, which should be selected to meet your needs, perhaps the most important consideration is the interface type and speed - as this will significantly affect how quickly you can move files between devices.
Solid state devices (USB Memory stick, a.k.a. “thumb-drive” and SSD mass storage) are considerably more robust and have lower power needs than portable Hard Disk Drives (HDD). Portable HDDs are relatively cheap; cost per MB/GB of storage is considerably lower than SSD.
HDD storage is notoriously power hungry. These must have their own source of power, or you will need to use a powered USB hub to provide adequate power in most instances.
Interface
Solid State USB memory devices will often have a USB Type-A or USB-C physical connector. Unless all the devices to which you intend to connect your stage device (e.g., iPad, Mac, PC) all have a USB-C, you may find the USB Type-A is more flexible. High capacity SSD storage devices often present a USB-C interface.
You are likely to encounter USB 2.0, 3.0, 3.1 and 4.0 - the latter only being available with some USB-C devices. Interface speed increases dramatically as you move upwards in the scale. You should be looking for storage devices that present at least USB 3.0 - or USB 3.1 (or higher) by preference. Large (or many) files will take an age to be copied to or from USB 2.0 storage, although for small files the difference in time will be unnoticeable.
Speed
Although related to the interface type, the limiting factor for speed of transfer is actually that of the storage medium itself. There are always two key numbers - read and write - and manufacturers always tend to concentrate on the higher number in their specifications and marketing (i.e., read speed).
In reality, it’s the lower number (write speed) that you need to concentrate your attention, selecting a device with a write speed that is as high as practicable. Be aware, write speed is proportional to cost. For practical purposes, USB 3.1 or higher devices will usually have respectable write performance - with read speeds often approaching the practical maximum of the interface itself.
Capacity
Very high capacity drives are not necessarily the best choice. Cost goes up, performance goes down. If something goes wrong, or the device is corrupted, larger drives will cause loss of more (potentially irreplaceable) data. Solid State drives are physically small, so having several drives offers some advantage in being able to keep independent copies of important data if needed.
Formatting
To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32 or exFAT (i.e., FAT64) with a single data partition; APFS or HFS+ are also supported by iPad, but not linux.
FAT-based formats are universally supported by most computer systems. Proprietary Apple formats (APFS and HFS+) are not supported by PC. Likewise, Microsoft proprietary NTFS is not usable with iPad and Mac.
Many (if not most) USB storage devices are factory formatted to FAT32 or exFAT, usually with a single partition. If a storage device needs to be re-formatted or re-partitioned, you will need either a PC or Mac to perform this action. iPad has no facility to partition/format external storage devices.
USB Hub
Your iPad Pro M1 has a single USB-C port with which to connect power, storage and any other USB accessory. You will likely find that a USB-C hub will be invaluable - as the hub provides flexibility in simultaneous connection of Power and storage devices.
Many USB hubs incorporate USB Type-A and USB-C ports (providing flexibility in choice of attached storage), plus others - often including a mix of Ethernet LAN, HDMI, VGA, SD, microSD.
I hope this information and insight proves to helpful in informing your eventual choice of storage debices.
